The median survival time of AKR mice transplanted with syngeneic lymphoma cells was increased by three weekly injections of C. parvum prior to tumor transplant. The enhanced tumor resistance is correlated with the ability of C. parvum to activate the peripheral blood monocytes, but not the peritoneal macrophage. Local suppression of transplanted lymphoma by macrophages did not prolong the survival of mice if the blood monocytes were not activated. There is preliminary evidence suggesting that macrophage resistance clone(s) of AKR lymphoma exist. Patients with acute nonlymphocytic leukemia as a group had significantly fewer natural killer cell activities against HL-60, a human progranulocytic leukemic cell line, as compared to a group of normal individuals. The immunotherapy with neuraminidase treated allogeneic myeloblasts did not alter the natural killer cell activities in the patients. The natural killer cell activities appeared to correlate well with the disease status in bone marrow.